1. Field of the Invention
The present invention relates to an improvement of a melt-type thermal transfer printing apparatus realizing a multi-gradational printing image using a heat fusible ink and a printing sheet used for this melt-type thermal transfer printing apparatus.
2. Prior Art
Published Japanese Patent Application No. 07125468 A/1995, entitled "Thermal Transfer Recording Sheet and Thermal Transfer Printer Using the Same", proposes a thermal transfer printer and a thermal transfer printing sheet used for this printer.
The technique disclosed in this prior art relates to a recording of a multi- gradational image that is realized by using a porous recording medium which comprises a porous layer having a pore diameter of 1 to 10 .mu.m formed on a base material made of a plastic such as a synthetic paper or a polyester.
However, when the above-described conventional porous recording medium is used as a printing sheet, there is a problem that the touch is different from that of a plain paper or that the price is expensive.
In general, a plain paper made of pulp is not suitable when used for realizing a porous layer because a pore diameter of the resultant porous layer becomes relatively large.
In view of practical aspects, a development is earnestly expected in the field of a printing technology applicable to papers having a porous layer whose pore diameter exceeds 10 .mu.m.